It has become desirable to employ non-impact xerographic or other photosensitive printers for text and graphics. In a xerographic printer, an electrostatic charge is formed on a photoreceptive surface of a moving drum or belt, and selected areas of the surface are discharged by exposure to light. A printing toner is applied to the drum and adheres to the areas having an electrostatic charge and does not adhere to the discharged areas. The toner is then transferred to a sheet of plain paper and is heat-fused to the paper. By controlling the areas illuminated and the areas not illuminated, characters, lines and other images may be produced on the paper.
One type of non-impact printer employs an array of light emitting diodes (commonly referred to herein as LEDs) for exposing the photoreceptor surface. A row, or two closely spaced rows, of minute LEDs are positioned near an elongated lens array so that their images are arrayed across the surface to be illuminated. As the surface moves past the line of LEDs, they are selectively activated to either emit light or not, thereby exposing or not exposing, the photoreceptive surface in a pattern corresponding to the LEDs activated.
To form good images in an LED printer, it is desirable that all of the light emitting diodes produce controlled light output when activated. This assures a uniform quality image all the way across a paper for black and white printing, and control of exposure for grey scale printing. The light output from an LED depends on a number of factors including current and temperature.
Light emitting diodes for print heads are formed on wafers of gallium arsenide or the like, suitably doped to conduct current and emit light. Long arrays of LEDs are formed on a wafer which is cut into separate chips each having an array of LEDs. A row of such chips are assembled end-to-end on the print head. The LEDs are driven by power supplies on nearby integrated circuit chips. Typically, an integrated circuit chip provides constant current for all the LEDs on an LED chip, or sometimes two integrated circuit chips each provide current for half of the LEDs on an LED chip.
The light output of an LED varies with temperature. It is important for some applications such as grey scale printing that the light output be uniform over time. For black and white printing it is important that the light output be reasonably uniform across the width of the printhead. Depending on the past history of power dissipation in the printhead, some LED chips may be much warmer than others, thereby causing a significant nonuniformity in light output. Hence, exposure of the photosensitive medium temperature may also vary with time.
The light output varies as much as -0.9% per degree Centigrade and since appreciable temperature differences may occur during operation of the printhead, there may be substantial exposure differences as temperature varies across the head and as a function of time. It is therefore desirable that compensation be provided for temperature changes that may occur during operation of an LED printhead.